The present invention relates to a laser apparatus and, particularly, to an improvement of the laser beam quality of a high power laser apparatus.
FIG. 24 is a cross sectional side view of a conventional laser apparatus having an unstable resonator such as shown in "Laser Handbook", 1979, North-Holland Publishing Company. In FIG. 24, a full reflection mirror system composed of a collimating mirror 1 which is a concave mirror and a convex, expanding mirror 2 are arranged opposite the collimating mirror Reference numeral 3 depicts a laser medium which may be a gas medium capable of being excited by electric discharge when an associated laser is a gas laser such as CO.sub.2 laser or a glassy medium to be excited by such as a flash lamp when the laser is a solid state laser such as a YAG laser. Reference numeral 4 depicts a window mirror which has a non-reflective coating 5 and, together with the full reflection mirror and an enclosure 6, defines an optical resonance cavity in which laser beam 7 is generated which is derived through the window mirror 4 surrounding the expanding mirror 2 as an output beam.
In operation, the mirrors 1 and 2 constitute an unstable resonator and laser beam reflected by the mirror 2 and diverging therefrom is amplified by the laser medium 3 and then collimated by the collimating mirror 1 to a parallel beam which is derived from the peripheral portion of the window mirror 4 as an annular beam 8. Since the output beam 8 is substantially coherent, it can be used effectively for cutting or welding steel plate etc. after being condensed by a suitable lens system to a pattern having beam energy density highest in a center portion thereof.
The degree of condensation depends upon a ratio of an inner diameter of the annular laser beam pattern to an outer diameter thereof, i.e., magnification factor ( referred to as "M value" hereinafter), and the larger the M value, i.e., the higher the energy density of the center portion of the condensed beam, provides the better condensation. However, since, when the M value is made too large, an oscillation efficiency is substantially degraded, an upper limit of the M value is practically in the order of 2. Thus, it is impossible to make the M value closer to a maximum value with which the maximum condensation is obtained. Further, since the window mirror 4 is non-uniformly heated by the output laser beam having the annular pattern, a non-uniform internal stress is produced in the mirror 4 by which phase distribution of the output beam passing therethrough is changed causing the condensation to be degraded.